A World Science Festival panel on Cyclic Cosmology hosted by Briane Greene featuring Anna Ijjas, Paul Steinhardt and Peter Galison

A wide range of empirical evidence supports the notion that the universe has been expanding and cooling for the last 14 billion years. However, the idea that it began with a bang is pure speculation based on extrapolating back in time, assuming equations remain valid under conditions far beyond where they have been tested. In this talk, Paul Steinhardt explains why it may be time to jettison the Big Bang. Namely, a series of recent advances strongly suggest that the only way to describe the remarkable homogeneity and isotropy observed on large scales may be if the universe first underwent a period of ultra-slow contraction. In that case, it is essential to replace the bang with a ‘bounce’ — a smooth transition from contraction to a dense, hot universe that proceeds to expand and cool. Among possible implications is a novel kind of cyclic theory of cosmic evolution.

An introduction to a new kind of cyclic model that avoids a common error of past cyclic models that evolution should be periodic in the scale factor, a(t). Instead, we argue, cyclic evolution must be periodic in the Hubble parameter (or Hubble radius), in which case a(t) grows from cycle to cycle and there is no big crunch. In terms of H(t), the universe is cyclic, but, in terms of a(t), the universe is self-similar and expanding from cycle to cycle.

This talk presents new numerical results demonstrating that slow contraction is a much more powerful smoothing mechanism than previously thought and other surprising behaviors of contracting cosmological phases.

p id="2018"> This talk explains how models that replace the big bang with a classical, non-singular transition from contraction to expansion can address many of the fundamental problems of cosmology.

p id="2018"> In this talk, Anna describes newly discovered, smooth, non-singular bounce solutions. In particular, she focuses on how adapting insights and tools from mathematical relativity has been crucial for these constructions and why it will also be a central focus looking ahead.

p id="2018"> In this talk, Frans gives an introduction to numerical solutions of the Einstein equations, focusing on their application to bouncing cosmologies. He reviews some work that has been done and discusses some of the challenges going forward that will need to be overcome to fully investigate generic, non-perturbative regimes in non-singular bounce models.